1. Field of the Invention
The present invention relates to a color image forming apparatus and, more particularly, to an apparatus which records color image information by optically scanning the surfaces of a plurality of image carriers with a plurality of corresponding light beams output from a plurality of scanning optical devices, and is suitable for an apparatus such as a laser beam printer, color digital copying machine, or the like having a color electrophotography process.
2. Related Background Art
In a conventional scanning optical device used in a laser beam printer (LBP), digital copying machine, or the like, a light beam which is optically modulated in accordance with an image signal and is output from a light source means is periodically deflected by a light deflector such as a rotary polygonal mirror, and is focused to form a beam spot on the surface of a photosensitive recording medium (photosensitive drum) by a scanning optical element (imaging element) having f-.theta. characteristics. The beam spot is scanned on that surface to record an image.
FIG. 1 is a schematic sectional view showing principal parts of a conventional scanning optical device of this type.
Referring to FIG. 1, a divergent light beam emitted by a light source means 91 is converted into a nearly collimated light beam by a collimator lens 92, and the light beam (light amount) is limited by a stop 93. Then, the light beam enters a cylinder lens (cylindrical lens) 94 having a predetermined refractive power in only the sub-scanning direction. Of the nearly collimated light beam that enters the cylinder lens 94, light components in the main scanning section directly emerge as a nearly collimated light beam. On the other hand, in the sub-scanning section, light components are focused to form a nearly linear image on a deflection surface (reflection surface) 95a of a light deflector 95 that comprises a rotary polygonal mirror. Note that the main scanning section indicates a light beam section temporarily formed by a light beam deflected and reflected by the deflection surface of the light deflector. On the other hand, the sub-scanning section indicates a section which includes the optical axis of an f-.theta. lens and is perpendicular to the main scanning section.
The light beam deflected and reflected by the deflection surface 95a of the light deflector 95 is guided onto a photosensitive drum surface 98 via a scanning optical element (f-.theta. lens) 96 having f-.theta. characteristics. By rotating the light deflector 95 in the direction of an arrow A, the light beam scans the photosensitive drum surface 98 in the direction of an arrow B. In this way, an image is recorded on the photosensitive drum surface 98 as a recording medium.
FIG. 2 is a schematic view showing principal parts of a color image forming apparatus which records image information in units of colors on different photosensitive drum surfaces by simultaneously using a plurality of scanning optical devices described above, thus forming a color image.
Referring to FIG. 2, the color image forming apparatus comprises scanning optical devices 101, 102, 103, and 104, photosensitive drums 111, 112, 113, and 114 serving as image carriers, developers 121, 122, 123, and 124, and a conveyor belt 131. In the color image forming apparatus shown in FIG. 2, the four scanning optical devices (101, 102, 103, 104) are placed at neighboring positions, respectively correspond to C (cyan), M (magenta), Y (yellow), and B (black), and in parallel record image signals on the surfaces of the photosensitive drums 111, 112, 113, and 114 to print a color image at high speed.
In such a color image forming apparatus, since an image is formed by superposing a plurality of scanning lines, it is especially important to minimize scanning line errors (also referred to as "registration errors" hereinafter) among the respective colors. For this purpose, the scanning optical device is required to:
(a) shorten the optical path length of the optical system so as to minimize scanning line errors due to initial conditions (manufacturing errors, changes upon installation, and the like) and environmental conditions (temperature elevation and the like); and PA1 (b) reduce the incidence angle to the surface to be scanned in the main scanning plane so as to minimize errors in magnification in the main scanning direction due to attachment errors of a photosensitive drum (or a cartridge containing it) and decentered rotation of the drum itself. PA1 the fourth optical element has a plastic toric lens, and the fifth optical element has a compound optical element that includes a combination of a cylinder lens and diffractive optical element, PA1 the fourth optical element has a plastic toric lens and first diffractive optical element, and the fifth optical element has a compound optical element that includes a combination of a cylinder lens and second diffractive optical element, PA1 the fifth optical element is formed of plastic, or a maximum exit angle .theta..sub.p the light beam deflected by the deflection element makes with the optical axis of the third optical element is set to fall within a range from 40.degree. to 60.degree.. PA1 the change in aberration in the main scanning direction is a change in focus, or the like. PA1 the refractive portion of the optical element has a plastic toric lens having different powers in the main scanning and sub-scanning directions and a cylinder lens having a power in the main scanning direction, the diffractive portion of the optical element has a diffractive optical element having different powers in the main scanning and sub-scanning directions, and the cylinder lens and diffractive optical element are combined to form a compound optical element, or PA1 the refractive portion of the optical element has a plastic toric lens having different powers in the main scanning and sub-scanning directions and a cylinder lens having a power in the main scanning direction, the diffractive portion of the optical element has a first diffractive optical element having a power in the main scanning direction and a second diffractive optical element having different powers in the main scanning and sub-scanning directions, the first diffractive optical element is placed in the vicinity of the toric lens, and the cylinder lens and second diffractive optical element are combined to form a compound optical element.
Japanese Patent Application Laid-Open No. 7-128603 discloses an example wherein a scanning optical system for a scanning optical device used in a color image forming apparatus is constructed by a glass lens and glass cylinder mirror. In this reference, although the incidence angle of a light beam onto the surface to be scanned is small, since the optical path length of the optical system is as large as about 400 mm, the device is readily influenced by the above-mentioned variations, and the entire device becomes bulky.
Japanese Patent Application Laid-Open No. 8-76011 discloses an example in which a scanning optical system for a scanning optical device is constructed by a single toric lens. In this reference, although the optical path length of the optical system is short, the incidence angle of a light beam onto the surface to be scanned is as large as .theta..sub.i =30.degree., and the device is readily influenced by decentered rotation of the photosensitive drum.